# Friday, 18 November 2005

More on SPARC

Yesterday we saw briefly the architecture of SUN SPARC processors, and how the presence of a big register array and its organization into register windows affected the calling convention and the assembly programming model.
The idea seemed clever, and C programs should be affected very positively by this design (we know that function calls are a great performance issue on x86). However, its is with a reason that the SPARC-Solaris platform was called Slowlaris.. =)
The register window was designed without consideration of a real word scenario: the presence of all those register is a terrible slowdown on multitasking operating system; when a context switch happens, all the register of the thread being switched off must be dumped into memory. It's not like on x86, where only general purpose registers shuld be pushed in memory; the other ones are "hidden", and used cleverly by the processor.
For SPARC processors, the number of registers is (for a 8 windows processor) 136, i.e. about 1Kb. With the increasing of CPU frequencies, memory bandwidth is a major bottleneck, and with a slice time of 50 ms (a quite high slice time) the memory wasted is about 20Kb every second.
It may seem not too much, but as an example I have read that IPC would be 5 times slower on a Sparc processor than on
a comparable 8-register processor, if all  Sparc registers are saved and restored on context switch.
Algorithms for lazy context switch have been proposed, but those are applicable only through a special designed kernel or through harware modifications.
Hence the "friendly" name Slowlaris for the SPARC-Solaris pair.

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